How to determine the element in the place of which another element was placed?

Question

JavaScript

Hello. There are 2 elements, both can be moved in absolute position on the browser's client area. If I move the 2nd element and put the 1st element in place, how can I programmatically determine the 1st element, can anyone come across such a task?
I will add that there can be more than 2 elements. I just wrote it as an example to state the essence of the issue.